Within bosonic string theory, in the closed string sector, the spectrum contains three massless states \(g_{\mu \nu }\) , the graviton, \(B_{\mu \nu }\) , the Kalb-Ramond (KR) antisymmetric field and \(\phi \)  the dilatonIn superstring theory any tachyonic states present in the bosonic formulation are eliminated.. In an effective field theoretic approximation to strings, the low energy excitations of strings, have their own fields. Strings do not have to be formulated in flat backgrounds and in particular they can be formulated on backgrounds formed from gravitons, KR fields and dilatons. For consistency of the world sheet formulations of string theory it is necessary to have conformal invariance [1]}, [2]}. This leads to the vanishing of beta functions calculated through the renormalisation group. Effective actions can be constructed for these background fields so that their equations of motion are equivalent to the vanishing of beta functions. The calculations of these beta functions can be done in a Regge slope (\(\alpha ^{\prime } \)  ) expansion.
